An electric machine converts electrical source energy, e.g., from a battery pack or other direct current (DC) source, into rotational mechanical energy. Electromechanical energy conversion occurs primarily via interaction between the opposing magnetic fields of multiple permanent magnets and electromagnets/coils of the rotor and stator portions of the electric machine. Resultant motor torque may be harnessed as needed in a host of useful applications. Many types of electric motors can also be configured and/or controlled to generate electrical energy for recharging a battery pack and/or for powering one or more electrical systems. Such dual-use electric machines are commonly referred to as motor-generator units (MGUs).
An MGU may be configured as an alternating current (AC) or as a direct current (DC) electric machine. In a brush-type DC motor, the battery pack provides DC electrical energy to multiple rotor coils via a commutator. The commutator causes the electrical current conducted within the rotor coils to switch polarity in conjunction with rotation of the rotor. Brush-type DC motors are useful for many applications. However, because of sustained direct contact with the brushes, such machines tend to experience relatively high friction losses.
In a brushless-type DC (BLDC) electric motor, electronic commutation is used to replace the various mechanical commutators and brushes of the brush-type DC motor. This in turn helps to reduce friction losses, which can make the BLDC motor a potentially attractive option for use in certain applications. However, conventional BLDC motor designs may remain less than optimal when used in certain power- or torque-intensive applications.